This invention relates to a metal assembly for bonding dissimilar metals by hot rolling, a method of bonding said dissimilar metals, and a method of forming said metal assembly.
Bonding of dissimilar metals such as nickel and carbon steel by hot rolling is known. U.S. Pat. Nos. 2,018,725 and 2,745,172 describe heating a metal assembly comprising an assembled slab of dissimilar metals before hot rolling. During heating of the assembly, the air and any other gases in the assembly expand and are allowed to escape through vents. U.S. Pat. No. 2,745,172 discloses filling of the vents with refractory material to prevent air from returning to the inside of the assembly. These patents do not deal with avoiding the problem of oxidation of the metal surfaces to be bonded by evacuation of the metal assembly.
Such evacuation by applying a vacuum to prevent the metals from being oxidized on heating is a later development and is for instance disclosed in U.S. Pat. No. 2,758,368. According to this evacuation method, air and any other gases are exhausted from dissimilar metals sealed from the outside atmosphere. After a substantial vacuum is produced, the evacuated assembly is sealed off to prevent access of any air. The sealed assembly is then heated and worked by pressing or rolling.
U.S. Pat. No. 2,758,368 does not disclose means for venting air remaining due to imperfect evacuation. It is known that conventional evacuated assemblies expand, sometimes to twice the original thickness, on heating before hot rolling. They often rupture during the initial stages in the rolling mill resulting in clad materials of poor quality and in yield loss.
It is an object of the invention to allow venting of trapped gases during the heating step before hot rolling.
It is a further object of the invention to reduce bonding failure in bonding dissimilar metals.